The present invention generally relates to tone arm assemblies of the type employed with sound reproduction equipment, such as phonograph assemblies and the like. In particular, the present invention is directed to a novel stabilization and damping system to provide correct geometric stabilization of unipivot tone arm assemblies such as described in U.S. Pat. No. 4,587,646, Tone Arm Assembly, issued May 6, 1986 to this applicant.
A unipivot tone arm assembly is supported on a single pivot bearing assembly having a pivot point coinciding with an imaginary horizontal line extending through a center of gravity of the tone arm assembly, with the horizontal line extending perpendicular to a longitudinal axis extending through the tone arm assembly from a pick-up cartridge position at one end to a counterweight positioned at an opposite end thereof.
Phonograph records, also referred to as Long Playing (LP) records, are cut with a tangential cutting stylus which traverses the recording disk at precisely right angles to the grooves. Ideally, this is also the path that the reproducing tone arm will follow. However, with tangential tone arms, there are related problems regarding mechanical difficulties such as freedom from sticking and adequate energy dissipation in a low-mass system such as is dictated by the typical tangential tracking system.
When playing a warped phonograph record, the tracking force (stylus contact pressure) changes with the vertical position of the tone arm. The effect of these changes is that the stylus tip tracing the phonograph record groove walls changes. This causes an immediate change in the reproduced sound negatively affecting such parameters as frequency response, channel separation and other spatial characteristics etched into the phonograph record groove.
There are three types of balance for a unipivot tone arm assembly: unstable balance; stable balance; and neutral balance. Unstable balance positions the pivot fulcrum below the center of gravity and is unsuitable for any tone arm application. A tone arm with unstable balance will never track properly in the vertical plane. A tone arm with unstable balance will have a decreasing tracking force as the arm is lifted as when playing a warped record. The tone arm will have a tendency to continue traveling upwards at the peak of the warp, possibly even leaving the phonograph record surface entirely, rather than staying in the proper playing position.
Stable balance positions the pivot fulcrum above the center of gravity of the moving system. This kind of balance is useful in scales or other weight measuring systems. With this system any deflection of the moving system creates an immediate and equally strong opposing force with tries to right the assembly to its original rest position. When playing a warped record, the tracking force will constantly change with the vertical position of the tone arm. This in turn will change the stylus rake angle (the angle defined by a vertical line from the centerline of the stylus tip relative to the phonograph record groove) as the suspension mechanism of the cartridge is flexed due to these changing forces. The net effect of this will be that the micro-dimensions of the stylus tip tracing the phonograph record groove walls will also change angle. This will cause an immediate change in the reproduced sound, negatively affecting such functions as frequency response, channel separation and other spatial characteristics etched into the record groove. Unipivot tone arm assemblies are traditionally set to stable balance by adjustment of the counterweights.
Neutral balance, where the pivot point and the center of gravity share a common line, is the preferred method of balance for a tone arm. The up and down motion from the stylus riding over warped phonograph records will not create a strong restoring force with neutral balance. Instead, only the constant, downward tracking gravity-applied force by displacement of the balancing weight will be observed. The stylus contact pressure remains constant regardless of whether the tone arm is tracking a flat phonograph record or one with warps which raises the tone are upward. The stylus rake angle will remain unaffected resulting in more accurate tracking of the grooves. However, with neutral balance there is no restoring force around the lateral axis to maintain proper stylus position with respect to the phonograph record. The lack of lateral stabilization and damping has been the drawback to setting unipivot tone arms in the preferential neutral balance configuration. The present version of this invention provides the lacking lateral stabilization and damping through a novel magnetic stabilization system.
A tone arm with an angled (offset) head shell will exhibit a tendency to swing toward the center of a rotating record. This is caused by the vectors of the rotating forces involved in the overhang (distance of the stylus tip extending past the center of the record) combined with the offset angle. Anti-skate systems are devices which apply a countering force to the tone arm effectively canceling out the natural forces tending to “skate” the arm toward the center. The goal of an anti-skate system is to apply the correct amount of continually changing force at any position on the phonograph record. The magnetic stabilization system provides a unique method of applying anti-skating forces that vary according to the position of the stylus over the phonograph record.
As will become evident from a reading of the following specification and claims, the novel tone arm magnetic stabilization and damping system achieves a method of lateral stabilization achieving neutral balance, as well as anti-skating, and thus avoiding the undesirable effects associated with stable balance.